Method and Apparatus for Forming a Beam

ABSTRACT

A device for forming a beam of an antenna array, the device including: an antenna array having a plurality of spatially distributed elements; a processor for selectively switching said elements between first and second states wherein, in said first state, said elements are configured to receive an incoming signal; and a receiver operatively associated with said antenna array and said processor for generating a reference signal, mixing said incoming signal with a modified reference signal to generate a mixed signal and summing the mixed signal over a predetermined period to generate an accumulated signal, wherein said reference signal is modified prior to being mixed with said received signal such that said accumulated signal is indicative of the direction and magnitude of the beam of the antenna array.

FIELD OF THE INVENTION

The present invention relates generally to positioning systems and inparticular to subsystems for receiving positioning signals.

The invention has been developed primarily for forming a beam forreceiving positioning signals in a multipath environment and will bedescribed hereinafter with reference to this application. However, itwill be appreciated that the invention is not limited to this particularfield of use.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of the common general knowledge in the field.

As is known in the art, positioning technologies generally operate bymeasuring the time a signal takes to traverse from a signal source tothe receiving device. In most prior art applications, this measurementis taken by comparing the time at which a signal is sent with the timeat which the same signal is received. Common positioning systems such asGPS use three or more such signals and, using trilateration, calculatean object's position. Since the measurement calculations aretime-sensitive, a fourth signal is commonly required to ensure that theclocks of the source and the receiver are properly synchronised.

Multipath refers to the phenomenon whereby positioning signals arereflected off other objects, such as walls and furniture. This isespecially prevalent in an enclosed environment, such as indoors, but isalso a significant problem in built up areas, such as in cities.Simplistically speaking, reflected signals take longer to traverse froma source to a receiver, therefore affecting the accuracy of themeasurements. Also the receiver sees conflicting signals originatingfrom the same source, having different timing information. Some modernreceivers use selection algorithms to try to determine the mostappropriate signal to use in position determination. However, receiverstypically cannot differentiate multipath signals from the genuinepositioning signals to any high degree of accuracy.

Also known in the art are phased arrays, consisting of a number ofantenna elements that can be individually controlled to direct a beam.In a typical phased array, signals received at each element areindividually phase and gain manipulated, the exact manipulation requireddepending on the direction of the beam required. The resulting phase andgain manipulated signals from each element are then summed to obtain thedesired direction of the beam.

There are three main forms of phased array antennas in use today:

-   -   a) fixed beam forming;    -   b) sequential beam forming; and    -   c) simultaneous beam forming.

Fixed beam forming antenna arrays have a fixed phase relationshipbetween the elements and can only direct their beam in a singledirection. Since the direction of the beam is fixed, this type ofantenna cannot be used individually to track a moving signal source in apositioning system, such a satellite in a GPS application. A fixed beamforming antenna must be used in conjunction with some mechanical meansto steer the beam to the transmission source. Aside from reliabilityissues related to long term use of mechanical equipment, this mechanicalmovement must be coordinated with the direction in which the beam ispointed. This adds an additional source of potential error.

Sequential beam forming phased array antennas use discrete phase andgain circuitry attached to each element to form beams sequentially inmultiple directions. Discrete circuitry is required because each elementmust be individually controlled. Therefore, each element must haveaccess to its own suite of electronics, such as phase shifters, variablegain amplifiers, and associated control signals. Apart from theadditional costs arising for all the required discrete circuitry, andthe problems introduced in controlling this circuitry with the precisionrequired, this method is severely constrained when used in positioningsystems because only a single beam can be directed at a time. As notedabove, positioning systems such as GPS require the tracking of at leastthree signals, and to get the most accurate results these signals shouldbe tracked simultaneously. Sequential beam forming phased arrays aretherefore not suitable for use in positioning systems because theycannot track more than one signal simultaneously.

Simultaneous beam forming phased array antennas are also widely in use.Traditional simultaneous beam forming antennas use large arrays ofelements with complex circuitry to simultaneously form beams in multipledirections. These arrays require RF front ends and analogue-to-digitalconverters for each element, and a very complex array of digital logicin the baseband to combine all the element signals together. The size,power consumption, and cost of such arrays limit their use to very largeinstallations typically using hundreds of elements, for example inmilitary applications. Clearly, the size, complexity, power consumptionand cost of these systems make them unsuitable for use in positioningsystems.

Additionally for positioning systems using interferomic techniques, anyline biases or group delays introduced by the large scale parallelprocessing of traditional simultaneous beam forming antenna arrayscannot be tolerated. All these errors must be estimated and calibratedout of the system for centimetre positioning accuracies to be achieved.This is a non-trivial problem as these biases will change with circuittemperature, voltage, and component tolerances. Again, this makestraditional simultaneous beam forming antenna arrays unsuitable for usein high precision positioning systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

It is a further object of the present invention to create multiple beamssimultaneously.

It is a further object of the present invention to use a single RF frontend for simultaneously forming the multiple beams.

It is a further object of the present invention to form relativelynarrow beams using relatively large numbers of elements (generally morethan 32 elements), whilst minimizing electronic complexity.

It is a further object of the present invention to re-use standardpositioning receiver components/logic blocks to reduce powerconsumption, cost and complexity.

It is a further object of the present invention to provide a method ofsimultaneously forming multiple beams in different directions usingrelatively large numbers of elements (generally more than 32 elements)for positioning systems, whist obviating the need for complicatedcircuitry and calibration.

According to an aspect of the invention, there is provided a device forforming a beam of an antenna array, the device including:

-   -   an antenna array having a plurality of spatially distributed        elements;    -   a processor for selectively switching said elements between        first and second states wherein, in said first state, said        elements are configured to receive an incoming signal; and    -   a receiver operatively associated with said antenna array and        said processor for generating a reference signal, mixing said        incoming signal with a modified reference signal to generate a        mixed signal and summing the mixed signal over a predetermined        period to generate an accumulated signal, wherein said reference        signal is modified prior to being mixed with said received        signal such that said accumulated signal is indicative of the        direction and magnitude of the beam of the antenna array.

Preferably, the receiver includes at least one receive channel having acorrelator, wherein the operative association with the antenna array isprovided by the correlator. The operative association between theantenna array and the correlator is preferably provided by selectivelymanipulating the phase and/or gain of the reference signal insubstantial synchronisation with an element being switched to the firststate. The manipulation of the phase and/or gain is preferably achievedby respectively applying a phase and/or gain offset to the referencesignal, wherein the value of the phase and/or gain offset is determinedin dependence upon one of the elements being switched to the firststate.

Preferably, the correlator includes a carrier numerical controloscillator (NCO) and the reference signal is synthesised in the carrierNCO.

The value of the phase and/or gain offset is, in one embodiment,determined by the processor in real time. Alternatively, the value ofthe phase and/or gain offset is determined by retrieving a predeterminedvalue stored in a database that is accessible by the processor.

Preferably, in the first state, the element is active and in the secondstate, the element is inactive. The elements are preferably switchedbetween the first and second states in a predetermined sequence. Thepredetermined sequence preferably selectively excludes one or moreelements from being switched to the first state.

Preferably, the configuration of the antenna array is dynamicallyadjustable by switching one or more elements to the second state for theentire duration of said integration period.

The elements are preferably switched to the first state for asub-integration period, wherein the sub-integration period is less thanthe predetermined integration period.

Preferably, the predetermined integration period includes a plurality ofbeam forming slots (B-slots). An element is preferably switched to thefirst state for the entire duration of the B-slot.

In one embodiment, each B-slot is configured to access a respectiveaccumulator for storing the incoming signal and wherein each respectiveaccumulator is mixed with the modified reference signal to generate themixed signal.

Preferably, each B-slot within an integration period is dynamicallyadjustable such that one or more elements are selectively excluded frombeing switched to the first state.

The elements are preferably spatially distributed in a three dimensionalconfiguration such that the device can form beams in one or moredimensions.

Preferably, each receiver includes multiple receive channels and whereineach receive channel is adaptable to form at least one beam.

Elements switched to the second state are preferably configured to benon-resonant such that the effects of mutual-coupling are ameliorated.

A propagation delay is incurred between receiving the incoming signaland mixing the incoming signal, the propagation delay preferably beingaccounted for by delaying the modification of reference signal.

According to an aspect of the invention, there is provided a method forforming a beam of an antenna array, the method including the steps of:

-   -   a) selectively switching spatially distributed elements of said        antenna array from a second state to a first state, wherein, in        said first state, said elements are configured to sample an        incoming signal;    -   b) receiving, at said elements switched to said first state, an        incoming signal;    -   c) generating, in a correlator, a reference signal for        correlation with said incoming signal;    -   d) applying, in substantial synchronisation with said elements        being switched to said first state, a predetermined offset to        said reference signal to create a modified reference signal;    -   e) mixing said incoming signal with said modified reference        signal to create a mixed signal; and    -   f) accumulating said mixed signal over an integration period to        create an accumulated signal, wherein said accumulated signal is        indicative of the direction and magnitude of said beam of said        antenna array

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of an antenna array coupled with apositioning receiver according to one aspect of the invention;

FIG. 2 is a timing diagram showing the relationship between B-slots andthe integration period.

FIG. 3 is a flow diagram of the steps involved for forming beamsaccording to one aspect of the invention;

FIG. 4, is a schematic view of a modified correlator according to oneaspect of the invention; and

FIGS. 5a and 5b are schematic views of a two-antenna array according toone aspect of the invention.

PREFERRED EMBODIMENT OF THE INVENTION System Overview

According to the invention, there is provided a device and method forforming a beam of an antenna array in the direction of an incomingpositioning signal. Since the direction of the source of the positioningsignal is predetermined, the beam of the antenna array is formed in thedirection of the incoming positioning signal, maximising the gain ofthat incoming positioning signal while attenuating signals from otherdirections, thereby mitigating any unwanted effects of multipath.

Referring to FIG. 1, a device 102 for forming a beam in an antenna arrayincludes the antenna array 104, which has a plurality of spatiallydistributed elements 106. A processor 108 is coupled to the antennaarray 104 for selectively switching, via switches 110, the elements 106between first and second states in a predetermined pattern wherein, inthe first state, the elements are configured to receive an incomingpositioning signal and in the second state elements are configured tonot receive incoming positioning signals.

A positioning receiver is operatively associated with the antenna arrayand the processor for generating a reference signal, and is provided byselectively manipulating the phase and/or gain of the reference signalin substantial synchronisation with each element being switched to itsrespective first state. This creates a modified reference signal whichis then mixed with the received positioning signal to create a mixedsignal. This mixed signal is then accumulated over a predeterminedintegration period, such that the accumulated signal is indicative ofthe direction and magnitude of the beam of the antenna array.

In one embodiment, discrete components/logic blocks are used in acircuit utilizing mixers, oscillators and accumulators to produce therequisite accumulated signals before passing onto a positioning receiverfor subsequent processing.

However, the preferred embodiment is to incorporate the beam formingmethod of the present invention into a standard positioning receiver, asshown in FIG. 4. This is because much of the required circuitry forforming beams according to the present invention is already part ofstandard positioning receiver architecture, and only requires minormodification to allow the formation of simultaneous beams. This requiredcircuitry is embodied in the correlator which, as those skilled in theart would understand, includes mixers, oscillators and accumulators.These components of the correlator can be utilised in lieu of discretecomponents. This provides benefits such as cost and power savings, aswell added benefits in miniaturisation, integration and portability ofthe receiver device.

Referring again to FIG. 1, a positioning receiver 114 used in apositioning system is depicted according to the preferred embodiment.The positioning receiver 114 makes use of existing components includingat least one receive channel 116 having at least one correlator 118 thatis operatively associated with the antenna array 104 and the processor108. Each correlator 118 incorporates a carrier numerically controlledoscillator (NCO) for generating a reference signal. This referencesignal can have its phase and/or gain modified by the processor 108 insubstantial synchronism with each element being switched to its firststate, thus creating a modified reference signal. The receivedpositioning signal is subsequently mixed with this modified referencesignal to create a mixed signal. This mixed signal is then mixed with acode NCO reference signal, as per standard correlator processing, andsubsequently accumulated over a predetermined integration period tocreate an accumulated signal. The resulting accumulated signal istherefore indicative of the direction and magnitude of a beam 122 formedwithin the correlator 118 from the antenna array 104. At the end of eachpredetermined integration period the correlator lock loops operate, asper normal correlator operation, unperturbed by the phase and/or gainmanipulations.

Modification of the Reference Signal

After the reference signal is synthesised by the carrier NCO, it ismodified by selectively manipulating the phase and/or gain of thereference signal in substantial synchronism with elements being switchedto the first state. Specifically, the manipulation of the phase and/orgain is achieved by applying a phase and/or gain offset to the referencesignal, wherein the value of the phase and/or gain offset is determinedin dependence upon the respective elements that are switched to thefirst state and the desired direction the beam is to be formed.

The antenna array 104 is operatively associated with the correlator 118through insertion of the respective phase and/or gain offset within thecorrelator circuit. The operation of the correlator and the insertion ofthe phase and gain offsets are described in further detail below, withreference to FIG. 4.

In an embodiment, the value of the phase and/or gain offset isdetermined by retrieving a predetermined value stored in a database 120that is accessible by the processor. An offset table, such as the tableshown in the illustrative example below, is stored in the database 120and selectively accessible by the processor 108. Although a storeddatabase of predetermined offset values is the preferred method, it willbe understood by those skilled in the art that in alternativeembodiments, the phase and/or gain offsets are calculated in real timeby the processor 108.

Antenna Elements

In the embodiment shown in FIG. 1, patch elements are depicted in a 3×3array. However, it will be understood by those skilled in the art thatin other embodiments, monopoles, dipoles or other suitable antennaelements are utilised. It will be further understood that the disclosureherein applies equally to antenna elements deployed in antenna arrayshaving multiple dimensions. In fact, in many practical applications,antenna elements are spatially distributed in a three-dimensional shape.

Throughout this specification and in the claims, the “first” staterefers to when an element is active and the “second” state refers towhen an element is inactive. The actual implementation of the inactivestate varies depending on the type of element used, with the focusplaced on making elements non-resonant to mitigate the effects ofparasitics or mutual-coupling. For example, a ¼λ monopole element isswitched to open in the second state while a patch element is switchedto ground in the second state. In some embodiments, the switches alsoprovide a connection to a resistance, such as 50Ω in the second state.It will be appreciated by those skilled in the art that switching toother conditions, such as reactive loads, are also possible in thesecond state.

Beam Forming Slots

In the preferred embodiment, only one element 106 at a time is in thefirst state during the predetermined integration period, while all otherelements are in the second state. That is, for each beam formed, onlyone element 106 is able to receive the incoming signal at any instantwithin the integration period. Each element 106 is switched to the firststate for a so-called sub-integration period, which is less than thepredetermined integration period. In one embodiment, thesesub-integration periods are known as “beam forming slots” (B-slots).

The relationship between B-slots and the integration period is bestshown in FIG. 2. In the example of FIG. 2, B-slots 202 are each 1 μs inlength and the integration period is Nμs long. In essence, the length ofthe integration period is simply divided into a number of equal lengthB-slots. A B-slot is therefore simply a period of time, during which theassociated element that is switched to the first state receives anincoming positioning signal.

In one embodiment, elements 106 are dynamically assigned to a B-slot. Incertain applications, such as bright-side scan mode operation discussedbelow, only a subset of elements on an antenna array are required toform a beam. This can be achieved by setting the unwanted elements toremain switched to the second state for the entire duration of theintegration period, so signals are not received from those elements andare therefore not accumulated. Alternatively, the length of the B-slotscan be extended so that only a subset of elements are allocated aB-slot, and therefore only signals received from that subset of elementsis accumulated.

However, in another embodiment, the minimum number of required B-slotscorresponds to the number of elements 106 that are spatially distributedon the antenna array 104. For example, in an implementation where theantenna array only includes two elements, the minimum number of requiredB-slots is two. Each element 106 is switched to the first state for theentire duration of its allocated B-slot.

In a further embodiment, ten elements are spatially distributed in anantenna array and ten B-slots are provided, one for each element. Usingan integration period of 1000 μs, which is a typical integration periodof a standard GPS receiver, elements are switched to the first state fora period of 100 μs each, in a predetermined manner (such as sequentiallyor pseudo randomly). When the first element is switched to the firststate, the processor also determines the phase and/or gain offset thatneeds to be applied to the reference signal, corresponding to the firstelement's position within the array and the direction of the beamrequired by the positioning receiver, and applies the offsets to thereference signal for the entire duration of the first allocated B-slot.In the subsequent 100 μs B-slot, the second element is switched to thefirst state while the first element and all the other elements areswitched to their second states. Again, the processor determines thephase and/or gain offset corresponding to the second element's positionwithin the array and the direction of the beam required by thepositioning receiver, and applies that phase and/or gain offset for theentire duration of the second B-slot. In this example, which uses asequential switching scheme, the third element is switched to the firststate in the third B-slot while the other elements are switched to theirsecond states, and so on for the subsequent B-slots within thatintegration period. At the completion of the 1000 μs integration period,all ten 100 μs B-slots will be accumulated with the requisite phaseand/or gain offsets to produce the desired beam required by thepositioning receiver.

Individual elements 106 in the antenna array 104 can only access thereceiver when switched to the first state. Since the direction of thebeam to be formed is known a priori, it follows that the elements mustbe switched to the first state in substantial synchronisation with theappropriate manipulation of the phase and/or gain of the referencesignal in order for the beam to be formed in the desired direction.Also, in order to obtain the full benefit of the allocated B-slot, itfollows that the phase and/or gain manipulation must be applied to thereference signal throughout the entirety of the allocated B-slot.

Beam Forming Methodology

The steps followed to form beams using the device disclosed herein aregraphically represented in the flow diagram of FIG. 3. A description ofthe steps taken is provided below.

-   -   a) At step 301, one of the spatially distributed elements in the        antenna array is selected and switched to the first state for        the first B-slot.    -   b) At step 302, the element switched to the first state at step        301 receives an incoming signal.    -   c) At step 304, the incoming signal is sampled at the RF front        end of the antenna array.    -   d) At step 308, an internal reference signal is generated in the        correlator for mixing with the incoming signal.    -   e) At step 310, a predetermined offset is applied to the        reference signal, in substantial synchronisation with step 302,        to create a modified reference signal.    -   f) At step 312, the modified reference signal is mixed with the        received signal to create a mixed signal.    -   g) At step 314, the mixed signal is accumulated in the        accumulators to create an accumulated signal.    -   h) At step 306, the selected element is switched to the second        state, the next element is switched to the first state in the        next B-slot and the process starts again from step 301.    -   i) At step 316, after accumulating all the B-slots together at        the end of the integration period, a beam is formed in the        accumulators based on the value of all the B-slot signals.    -   j) At step 318, the carrier and code lock loops are updated        using the accumulated B-slot signals.

Correlator Operation

A GPS position receiver typically uses a logic block called a correlatorto correlate an incoming positioning signal with internally generatedreference signals. Referring to FIG. 4, in the correlator 118, anincoming positioning signal is mixed with two internally generatedreference signals. The first reference signal is a carrier referencesignal that is generated by the carrier NCO 408. Mixing the carrierreference signal with the incoming positioning signal generates an errorsignal representing a phase and frequency difference between the carrierreference signal and the incoming signal. The second reference signal isa code reference signal that, in this embodiment, is generated by thecode NCO 416. Once the incoming positioning signal has been mixed withthe carrier reference signal, the incoming positioning signal is mixedwith the code reference signal, which generates an error signalrepresenting the time delay between the code reference signal and theincoming positioning signal.

For simplicity, FIG. 4 only shows a single receive channel of apositioning receiver. However, those skilled in the art will appreciatethat modern receivers typically include more than a single receivechannel, with each channel typically including more than one correlator.

In FIG. 4, the incoming positioning signal is received at the input 402and stripped of the carrier component by mixing, in mixers 404 and 406,the incoming signal with a reference carrier signal to produce in-phase(I) and quadra-phase (Q) sampled data. The reference carrier signal issynthesised in the carrier NCO 408 and the discrete sine and cosinemapping functions 410 and 412 respectively. This stripping processproduces I and Q signals as shown. In operation, the carrier NCO iscontrolled by the carrier lock loop 414. The objective of the carrierlock loop is to keep the phase error between the reference signal andincoming positional signal at, or as close as possible to, zero. Whenthe phase error is zero, the signals are said to be “phase-locked” andthe I signals are at a maximum while the Q signals are nearly zero. Thisoperation is also called “phase lock loop” (PLL) operation.

The I and Q signals are then correlated with a reference code signalthat, in this embodiment, is synthesised in the code NCO 416. For thesake of simplicity, only one reference code signal is synthesised inthis embodiment. However, those skilled in the art will recognise thatin most positioning receivers, more than one code reference signal issynthesised. For example, in one application, three code referencesignals—early, prompt and late signals—are synthesised and separatelycorrelated with the I and Q signals respectively.

The correlator 118 mixes an internally synthesised code reference signalwith the incoming I and Q signals in the mixers 418 and 420. Inoperation, the code NCO 416 is controlled by the code lock loop 426. Theobjective of the code lock loop is to keep the time error between theinternally generated code reference signal and incoming code positioningsignal at, or as close as possible to, zero. When the time error iszero, the signals are said to be “code-locked”. This operation is alsocalled “delay lock loop” (DLL) operation.

That is, the operation of the code lock loop 426 is similar to thecarrier lock loop 414. When the reference signal code phase iscompletely aligned with the incoming positioning signal code phase,maximum correlation is attained.

The resultant mixed signals are then integrated in the accumulators 422and 424 over an integration period, providing I_(p) and Q_(p) signals,which are subsequently accessed by the processor for tracking loopoperation.

The integration period refers to the length of time over which thereceived signal is accumulated, and is traditionally determined based ona satellite's pseudorandom code noise length or multiples thereof. InGPS, this code period is 1 ms, and thus the integration period in thereceiver is also often set to 1 ms or more.

Phase and/or Gain Offsets

In a preferred embodiment, phase and/or gain offsets for manipulatingthe phase and/or gain of the incoming positioning signal, is inserted atpoint 428, after the carrier reference signal is synthesised by thecarrier NCO 408 and before the synthesised carrier reference signal ismixed with the carrier component of the incoming positioning signal,completing the carrier lock loop 414. In this preferred embodiment thephase offsets are summed with the reference signal, and the gain offsetsare multiplied with the reference signal. Manipulation of the incomingpositioning signal is achieved by modifying the synthesised carrierreference signal within the integration period of the correlator,therefore not interfering with the normal operation of the carrier NCO408 or the carrier lock loop 414. The modified reference signal is thenmixed with the incoming positioning signal in the usual manner, and themixed signal is integrated in the accumulator over the integrationperiod to create an accumulated signal.

As known by those skilled in the art, the integration of a waveform issimply the summation of samples of that waveform over a given period oftime, in this case, the integration period. Therefore, the integrationof the resultant mixed signal (resulting from mixing the incoming signaland the reference signal) is simply the summation of samples of thatsignal over a period of time—which in one of the embodiments describedabove is the integration period of 1 ms.

In one embodiment, the incoming positioning signal is sampled at a rateof 75 MHz via an RF-frontend and the samples are then mixed with amodified reference signal, which is also synthesised at 75 MHz.Consequently, for a hypothetical system in which the integration periodis 1 ms comprised of 10 B-slots, each B-slot is of 100 μs in durationand therefore contains 7,500 samples of the incoming positioning signal.Each one of these 7,500 samples is sequentially mixed with a modifiedreference signal to form a mixed signal. The modified reference signalis based on a phase and/or gain offset applied to a reference signal,the reference signal being generated by the carrier NCO of thecorrelator. Specifically, for each block of 7,500 samples of theincoming positioning signal, which are synchronized with antennaelements being in the first state, the reference signal is modified byapplying a phase and/or gain offset to the output of the carrier NCO.This modified output is then multiplied (mixed) with the incomingpositioning signal samples. These mixed signals are then passed throughthe code NCO mixers, as per normal correlator operation, and then summedin the accumulators to form an accumulated signal. Therefore over theentire integration period of 1 ms, 75,000 samples, incorporating 10B-slot blocks of 7,500 modified samples each, are summed and stored inthe accumulators. In other words, these ten B-slots contain 7,500modified samples each of which are summed together in the accumulationprocess, and the 75,000 accumulated samples at the end of theintegration period are therefore representative of the desired beam.

Once the phase and/or gain manipulations are correctly applied to thereference signals and mixed with the signals received from therespective elements, the resultant mixed signal is then combined in theaccumulator (the summing process) to create an accumulated signal,forming the desired beam in the correlator. This accumulated signal isthen processed in the correlator as per normal PLL operation asdiscussed above. The carrier reference signal synthesised by the carrierNCO 408 does not change during the integration period, but is onlyupdated by the carrier lock loop 414 at the end of each integrationperiod. Therefore, modifications to the reference signal within theintegration period cannot be detected by the PLL or the DLL. The PLL andDLL operate as per normal, unaware of the manipulations taking place.

Through the embodiments described, the use of a conventional correlatoris adaptable to control the direction and the width of a unique beam percorrelator channel, thereby allowing multiple simultaneous beams to beformed. The number of beams able to be formed is equal to the number ofcorrelator channels available. This is because the correlator alreadycontains logic for mixing and integrating signals—these are simplyadapted for a use other than correlating.

Although the embodiments described above apply offsets to both the gainand the phase at point 428 in the correlator circuit, in otherembodiments, additional multipliers for applying gain offsets areprovided in other parts of the circuit. For example, multipliers can beadded in the In-phase and Quadra-phase paths between the carrier NCOmixers and the code NCO mixers to provide gain manipulation. Similarly,phase offsets can also be applied at other parts in the correlatorcircuit. For example, phase offsets can be added to the output of thecode NCO.

In the preferred embodiment, the phase and/or gain offsets for formingthe beam in any given direction are predetermined, and stored in thedatabase 120 and is accessible by the processor 108. The format of theoffset data can take many forms, such as an offset table. The processor108 determines the direction of the required beam, accesses the database120 to obtain the correct phase and/or gain offsets for each element ineach B-slot over the integration period to form the beam in the desireddirection, and inserts the necessary offsets at point 428 such that thebeam (122 in FIG. 1) is formed in the direction of the appropriateincoming positioning signal. As also noted, obtaining and inserting thephase and/or gain offsets must be substantially synchronous with theswitching of the respective elements into the first state so that thephase and/or gain is correctly manipulated over the integration period.

In one embodiment, an antenna array has 128 elements. Therefore, eachdirection has 128 phase and 128 gain entries in the offset table. If,for example, it is necessary for the beam to point north, the processorlooks up the table entry for north and steps through each of the 128phase and 128 gain entries for that direction over the integrationperiod to steer the beam north. In this embodiment, it is possible tosteer the beam in 2048 directions, with each direction having 128 phaseand 128 gain entries. Of course, in other embodiments, even moredirections are implemented. For this reason, it is preferable toimplement the phase and/or gain offset values in a tabular format, toalleviate processing overhead.

The embodiment noted in the above paragraphs is preferred because itminimises the processing power required. However, those skilled in theart will recognise that, especially given recent advances inmicroprocessor technology, the processor 108 is configurable tocalculate the phase and/or gain offsets as and when the offsets arerequired.

In a physical implementation of the present invention, each element isconnected by a transmission line 124 to a respective switch, which inturn feeds into a single RF frontend 126 to be downconverted and sent toat least one correlator. It should be noted that, in a preferredembodiment, the transmission line interconnecting the elements and theswitches are of equal length, to ensure received signals are phasecoherent through the antenna array feed system. However, in otherembodiments, differences in the lengths of the transmission line aretaken into account and corrected at the time of applying the phaseand/or gain offsets.

The interconnection between the antenna array 104 and the receivechannel 116, as well as the RF frontend 126, the electronics involved inthe correlator 118 and the actual switches 110 themselves, willinevitably cause delays. In one embodiment, this delay is measured to bearound 950 ns, but of course, those skilled in the art will appreciatethat the length of the delay will vary depending on the selectedhardware. Therefore, operation of the phase and/or gain manipulation inthe correlator cannot occur simultaneously with the switching of theelement to the first state, as this delay must be accounted for. Thatis, the manipulation of the phase and/or gain in the correlator must bedelayed by up to 950 ns in this embodiment.

In other embodiments, each antenna array contains over 60 elements withan integration period in the region of 100 μs. In such embodiments, theperiod of a B-slot is in the region of just 1 μs or 2 μs. Therefore, adelay of 950 ns—nearly 1 μs—is significant and must be accounted for.

The number of elements that the antenna array contains is one criterionfor forming narrow beams. Other, equally important criteria includeresolution of the offset table and the physical spacing of the elements.For each direction, each element must have associated offsets for phaseand/or gain. For example, in embodiments having 60 elements, tableentries for each direction in which a beam is to be formed must have 60gain offsets and 60 phase offsets recorded.

The physical separation of the elements is also important so as tocreate a phase difference between the elements. Effectively, thephysical separation of the elements allows a positioning signal to bereceived with inherently different phases. One half wavelengthseparation between elements provides maximum phasing with minimumgrating lobes. Manipulation of those phases, for example by mixing witha modified reference signal as noted above, allows for a beam to beformed in a desired direction.

In a particularly preferred embodiment, the elements 106 are spatiallydistributed in a configuration that is more than two dimensions suchthat the device can form beams in more than two dimensions. To a largeextent, the directions in which a beam can possibly be formed aredependent on the elements used. For example, a planar array consistingof patch elements will be able to form beams hemispherically and aplanar array consisting of monopoles will be able to form beams in aplane.

Bright-Side Scan Mode v Full-Scan Mode

It follows from the above discussion that at least two planar arraysconsisting of patch elements can be arranged so that the hemisphericalbeams are joined to form beams spherically. In this arrangement,therefore, beams can be formed in any direction.

In full-scan mode, all elements are switched between first and secondstates at least once within the integration period. That is, all theelements in the array receive incoming signals at least once within eachintegration period. Full scan mode-provides total flexibility in thatbeams can be formed in many different directions simultaneously,dependant on how many correlator channels are available.

However, when signals only illuminate one side of a three dimensionalantenna array, termed the “bright-side”, the information received fromthe other side of the antenna array, termed the “dark-side”, willpotentially be of little value. This situation occurs frequently interrestrial positioning systems, where transmitters are commonlydistributed on the horizon. In this case, dark-side elements are blanked(that is, the gain on these elements are set to zero) and onlybright-side elements are used. This effectively reduces the correlationduty cycle of the array by 50%, meaning that half of the correlationgain of the antenna array is wasted. For a positioning system spanning arelatively small area, this loss of correlation gain from the antennacan be tolerated, since the signal power of the transmitted signals isrelatively high. Therefore, full-scan mode is preferred as in small areasystems it is acceptable to trade-off maximum correlation gain foroptimum flexibility.

However, for larger geographical areas, where signals are transmittedover greater distances, correlation gain may become increasinglyimportant. In such instances, one way to increase the correlation gainof the antenna is to operate in bright-side scan mode. In this mode,elements on the bright-side are dynamically grouped together and theirrespective B-slot durations increased, such that only the bright sideelements are accumulated within the integration period. This results inan increase in correlation duty cycle and therefore an overall increasein correlation gain.

An Illustrative Embodiment

For illustrative purposes, the invention will now be described using thesimplest antenna array—an array having only two elements as shown inFIGS. 5a and 5b . However, those skilled in the art would be able toadapt the teachings herein to antenna arrays having many more elementsspatially distributed in multi-dimensional shapes without additionalinventive faculty.

In this illustrative embodiment, elements 502 and 504 are quarterwavelength mono poles. The two elements are placed a half wavelengthspatially apart from each other and signals are received at eachelement. When the two elements are summed together, the respectiveomni-directional gain patterns of the elements combine such that, from atwo dimensional top view of the elements, a figure-8 beam pattern isformed, as shown in FIG. 5a . In this configuration, an incomingpositioning signal from the broadside direction of the elements 502 and504 are in-phase, and hence magnified, while signals from the end-firedirection of the elements are out of phase, and hence cancelled.

Phase Manipulation

In the present invention, it is possible to rotate the figure-8 by 90°so that maximum gain is pointed in the end-fire direction, as shown inFIG. 5b . This is achieved by manipulating the phase and/or gain ofelement 502 and element 504 within an integration period of a positionreceiver. Element 502 and element 504 are each connected to a switch, sothat either element can be switched between first and second states andthe integration period over which the summation of the signal occurs issplit into two B-slots.

Since the phase separation between elements 502 and 504 is known, thephase of one of the elements can be manipulated so that incoming wavesfrom the end-fire direction are summed constructively instead ofdestructively. In this case, because the elements are half wavelengthapart, the phase manipulation required at element 504 is 180°. The phasemanipulations required for each direction is similarly calculated toconstruct an offset table. For the sake of simplicity, the gain offsetis set at 1 and the direction that the beam can be steered is limited toeither the broadside direction or the end-fire direction. An exampleoffset table incorporating these limitations is provided below.

Direction Broadside End-fire Element E502 E504 E502 E504 Phase  0°  0° 0° 180° Gain 1 1 1  1

To form a beam in the end-fire direction, elements 502 and 504 areswitched to the first state in a predetermined B-slot sequence withinthe integration period of the position receiver. In the first B-slot,element 502 is switched to the first state and the phase offset is keptat 0° while being accumulated in the accumulator—no manipulations arenecessary because this element is already at 0°. In the second B-slot,the phase of the incoming signal is desired to sum constructively atelement 504. Since element 504 has a receive phase of 180° relative toelement 502, a phase offset of 180° must be added to element 504, whilebeing accumulated in the accumulator, so that the received signal fromelement 504 becomes phase coherent with element 502. The two B-slots aretherefore summed together in the accumulation process and theaccumulated value at the end of the integration period is thereforerepresentative of the end-fire beam.

It will be understood by those skilled in the art that in the presentinvention, the figure-8 beam can be formed in any direction, dependingon the complexity of offset table.

For both elements in this simple example, a gain offset of 1 (unitygain) is multiplied with the incoming positioning signal and thereforedoes not modify the beam formed.

As noted above, the phase manipulation must be applied substantiallysynchronously to the switching of the elements to the first state;otherwise the gain pattern of the beam will not be formed correctly.

According to embodiments of the present invention, a positioning signalfrom, for example PRN1, commences transmission. After acquisition ofPRN1 at the positioning receiver, a carrier NCO within the positioningreceiver correlator synthesises a reference signal that is substantiallysimilar to the positioning signal.

The positioning receiver determined that PRN1 is in the end firedirection, and therefore a decision is made to form a beam in thatdirection. The processor looks up the offset table, and determines thatno offset is required for the first B-slot during the integrationperiod, which corresponds to element 502. Similarly, the processordetermines that a 180°, or ½λ, offset is required during the secondB-slot, which corresponds to element 504. After applying each offset tothe carrier NCO to create a modified reference signal, the incomingpositioning signal and the modified reference signal are mixed in amixer and accumulated in the accumulator, as per the normal operation ofa correlator. That is, with reference to FIGS. 1, 3 and 5, the incomingpositioning signal received at elements 502 and 504 is fed via switchesinto the RF frontend 126 and subsequently down converted and sampled inan analogue to digital converter. The received signal is then passed toat least one correlator 118.

The received positioning signal is stripped into its' in-phase andquadra-phase components, by mixing the received positioning signal witha carrier reference signal that is synthesised by the carrier NCO 408and the discrete sine and cosine mapping functions 410 and 412. Beforethe modified reference signal is mixed with the received positioningsignal, however, the processor determines that PRN1 is in the endfiredirection. For the first B-slot, in which element 502 is switched to thefirst state, the processor checks the table to determine if an offset tothe carrier NCO is required. In this case, an offset is not required andso no modifications to the reference signal are necessary. Theaccumulation is therefore allowed to proceed as per normal correlatoroperation. That is, the unmodified reference signal is mixed with thereceived positioning signal in mixers 404 and 406 to create a mixedsignal, then mixed with the code reference signal, and subsequentlyaccumulated in the accumulators 422 and 424.

For the second B-slot, the processor checks the table to determinewhether an offset to the carrier NCO is required. In this case, a phaseoffset of 180° is required. The offset is applied to the current carrierreference signal phase value to create a modified reference signal asthe second B-slot begins and element 504 is switched to the first state.The phase offset is applied continually to the carrier NCO valuethroughout the duration of the B-slot. The modified reference signal ismixed with the received positioning signal to create a mixed signal,then mixed with the code reference signal, and subsequently accumulatedwith the value of the first B-slot in the accumulators 422 and 424 tocreate an accumulated signal. The two B-slots are therefore summedtogether in the accumulation process and the accumulated value at theend of the integration period is therefore representative of theend-fire beam.

Note that the carrier reference signal synthesised by the carrier NCO408 does not change during the integration period, but is only updatedby the carrier lock loop 414 at the end of the integration period.

In embodiments discussed herein, the accumulated signal is created inthe accumulator over the entire duration of the integration period.However, in other embodiments, each B-slot is accumulated in its' ownindividual accumulator, the minimum number of accumulators correspondingto the minimum number of B-slots required. In these embodiments, theaccumulated signal is obtained from combining the signals in theindividual accumulators.

In the embodiments discussed, only one element can be in the first stateat any instant within the integration period. Therefore, when element504 is switched to the first state, element 502 is simultaneouslyswitched to the second state.

After traversing the code lock loop 426, the mixed signal is integratedin the accumulators 422 and 424, creating an accumulated signal. Sincethe manipulation to the signals occurs serially, the integration of themixed signal is, in effect, the summation of an infinite number ofmodified signals over the integration period. Therefore, the accumulatedsignal is representative of a new beam formed in the desired direction.

As described above, the antenna array 104 and switching circuit(switches 110) are coupled to a positioning receiver 114, which performsthe required PVT solution to determine the position of the receiver.Since the direction of incoming positioning signals are known for anygiven time, the beams formed in each correlator channel can be directedtowards those known directions to maximise the gain of the incomingsignal while attenuating signals from other directions, thereforemitigating the effects of multipath.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

In the claims below and the description herein, any one of the terms“comprising”, “comprised of”, or “which comprises” is an open term thatmeans including at least the elements/features that follow, but notexcluding others. Thus, the term “comprising”, when used in the claims,should not be interpreted as being limitative to the means or elementsor steps listed thereafter. For example, the scope of the expression adevice comprising A and B should not be limited to devices consistingonly of elements A and B. Any one of the terms “including”, “whichincludes”, or “that includes” as used herein is also an open term thatalso means including at least the elements/features that follow theterm, but not excluding others. Thus, “including” is synonymous with andmeans the same as “comprising”.

1. A device for mitigating multipath at a receiver, said deviceincluding: an antenna array having a plurality of spatially distributedelements; a processor for selectively switching said elements betweenfirst and second states, wherein said elements are configured to receivean incoming signal in said first state and configured to not receive anincoming signal in said second state; and a receiver operativelyassociated with said antenna array and said processor, said receiverconfigured to: generate a reference signal; modify said reference signalin substantial synchronisation with said elements being switched to saidfirst state; mix said incoming signal with the modified reference signalto generate a mixed signal; and sum said mixed signal over anintegration period to generate an accumulated signal, such that saidaccumulated signal is indicative of the direction and magnitude of saidbeam of said antenna array, wherein the modifications applied to saidreference signal are selected to form said beam in the known directionof a source of said incoming signal.
 2. A device according to claim 1,wherein said receiver includes at least one receive channel having acorrelator configured to generate said reference signal.
 3. A deviceaccording to claim 1, wherein the modification of said reference signalcomprises selective manipulation of the phase and/or gain of saidreference signal.
 4. A device according to claim 3, wherein saidmanipulation of said phase and/or gain comprises application of a phaseand/or gain offset to said reference signal, wherein the value of saidphase and/or gain offset is determined in dependence upon one of saidelements being switched to said first state.
 5. A device according toclaim 2, wherein said correlator includes a carrier numericallycontrolled oscillator (NCO) configured to synthesise said referencesignal.
 6. A device according to claim 4, wherein said processor isconfigured to determine the value of said phase and/or gain offset inreal time or by retrieving a predetermined value stored in a databasethat is accessible by said processor.
 7. A device according to claim 1,wherein said processor is configured to selectively switch any one ormore of said elements between said first and second states in apredetermined sequence.
 8. A device according to claim 7, wherein saidpredetermined sequence selectively excludes one or more of said elementsfrom being switched to said first state.
 9. A device according to claim1, wherein said receiver includes multiple receive channels, eachreceive channel being adaptable to form at least one beam.
 10. A deviceaccording to claim 1, wherein elements switched to said second state areconfigured to be non-resonant such that the effects of mutual-couplingare ameliorated.
 11. A device according to claim 1, wherein saidprocessor is configured to delay the modification of said referencesignal to account for a propagation delay incurred between receivingsaid incoming signal and mixing said incoming signal, such that saidsubstantial synchronisation is maintained.
 12. A method for mitigatingmultipath at a receiver having an antenna array, said method includingthe steps of: a) selectively switching spatially distributed elements ofsaid antenna array from a second state in which said elements areconfigured to not receive an incoming signal to a first state in whichsaid elements are configured to receive an incoming signal; b)receiving, through said elements switched to said first state, a signalfrom a source in a known direction; c) generating a reference signal; d)modifying said reference signal in substantial synchronisation with saidelements being switched to said first state to create a modifiedreference signal; e) mixing said incoming signal with said modifiedreference signal to create a mixed signal; and f) accumulating saidmixed signal over an integration period to create an accumulated signal,wherein said accumulated signal is indicative of the direction andmagnitude of said beam of said antenna array, and wherein themodifications applied to said reference signal are selected to form saidbeam in said known direction.
 13. A method according to claim 12,wherein said reference signal is generated in a correlator.
 14. A methodaccording to claim 12, wherein modifying said reference signal comprisesselectively manipulating the phase and/or gain of said reference signal.15. A method according to claim 14, wherein selectively manipulating thephase and/or gain of said reference signal comprises applying a phaseand/or gain offset to said reference signal, wherein the value of saidphase and/or gain offset is determined in dependence upon one of saidelements being switched to said first state.
 16. A method according toclaim 15, wherein the value of said phase and/or gain offset isdetermined in real time or by retrieving a predetermined value stored ina database.
 17. A method according to claim 12, wherein any one or moreof said elements are selectively switched between said first and secondstates in a predetermined sequence.
 18. A method according to claim 17,wherein a subset of said elements remain switched to said second statefor the entire duration of said integration period.
 19. A methodaccording to claim 13, wherein said correlator generates said referencesignal in a carrier numerically controlled oscillator (NCO).
 20. Amethod according to claim 12, wherein said receiver includes multiplereceive channels, and wherein said method further comprises the step ofadapting each receive channel to form at least one beam.
 21. A methodaccording to claim 12, wherein the modification of said reference signalis delayed to account for a propagation delay incurred between receivingsaid incoming signal and mixing said incoming signal, such that saidsubstantial synchronisation is maintained.